In the manufacture of many fabric-reinforced, molded rubber articles, especially automobile and truck tires, it is desirable to obtain strong adherence between the fabric and the rubber, and also high resistance to deterioration of the bond with flexing of the structure. In the case of pneumatic tires, the flex-life of the carcass is one of the most critical factors affecting the life of the tire. Long flex-life is dependent not only upon the inherent flexibility of the carcass structure and the strength of the bond between the tire fabric and the adjacent rubber, but also upon fatigue resistance under repeated flexing over long periods of time.
Many adhesives known to produce very strong bonds between rubber and fabric are entirely unsuitable for many rubber fabric structures because the bonds deteriorate or the fabric ruptures when the structures are subjected to repeated flexing. Thus, flex-life cannot be foretold from measurements of bond strengths alone.
It is possible to calender rubber into cotton fabrics in such a manner as to work the rubber between the fibers of the individual cotton cords or threads, thus obtaining a mechanical interlocking of the two materials. Such practices were followed for many years even though they were not entirely satisfactory. It should also be noted that such calendering procedures are substantially inoperative when applied to most continuous synthetic fibers, such as nylon, polyester and aramid fibers, which are formed into cords by a twisting process that leaves the cords virtually devoid of interstices into which the rubber can be forced by the calendering rolls. Since cotton fabrics are not widely used today as reinforcements for tires and other industrial rubber products, such calendering procedures are not a viable means for attaining needed levels of fabric to rubber adhesion.
The problem of obtaining satisfactory adhesion between rubber and cords of synthetic fibers was the principal obstacle that delayed the use of synthetic fibers in fabrics for pneumatic tire carcasses. The problem was finally solved to a degree, in the case of rayon, by the use of mixtures of natural latex and resorcinol or casein. The casein-latex adhesive was comparatively flexible, but the strength and character of the adhesive bond obtained left much to be desired. The resorcinol-formaldehyde latex mixture was developed and widely used in the tire industry, particularly with rayon fabrics. While fair adhesion was obtainable with that adhesive, it formed a relatively hard and stiff film on the fabric that seriously impaired its flexibility and flex-life and the adhesion between synthetic rubber and fabrics was nowhere nearly as good as desired.
U.S. Pat. No. 2,748,049 discloses an adhesive dip which is reported to strongly adhere to fabric and rubber and which is further reported to be relatively flexible. This dip is made utilizing a latex of a copolymer of a conjugated diolefin monomer, such as butadiene, and an unsaturated aldehyde monomer, such as methacrolein. However, dips made with such latices have never proven to be commercially viable.
In actual commercial practice, resorcinol-formaldehyde-latex (RFL) dips have been widely implemented by the tire industry for bonding synthetic fabrics to rubber. For instance, U.S. Pat. No. 3,525,703 discloses a water-based adhesive composition for bonding synthetic fiber material to rubber. The teachings of U.S. Pat. No. 3,525,703 specifically disclose the utilization of styrene-butadiene latex and vinylpyridine-styrene-butadiene latex in such water-based adhesive compositions.